Reword self-asserted VC in VP text to remove MUST.

The original statement was found to be untestable. This new text
provides explanatory text instead.

index.html

@@ -2667,10 +2667,9 @@ <h4>Presentations Including Holder Claims</h4>
 apply to self-asserted [=verifiable credentials=].
           </p>
           <p>
-When a self-asserted [=verifiable credential=] is secured using the same
-mechanism as the [=verifiable presentation=], the value of the
-`issuer` [=property=] of the [=verifiable credential=]
-MUST be identical to the `holder` [=property=] of the
+A [=verifiable credential=] in a [=verifiable presentation=] is considered
+self-asserted when the value of the `issuer` [=property=] of the
+[=verifiable credential=] is identical to the `holder` [=property=] of the
 [=verifiable presentation=].
           </p>
           <p>